In a gas turbine engine, inlet air is continuously compressed, mixed with fuel in an inflammable proportion, and then exposed to an ignition source to ignite the mixture which will then continue to burn in order to generate the combustion product.
The starting of a gas turbine engine is a complex process and generally includes two stages. In the first stage, the gas turbine engine is rotated by a torque provided by an external source, for example, by a starter. When a predetermined compressor pressure or speed is reached, fuel flow is injected at a controlled rate into the combustor to mix with the air flow and the mixture is exposed to an ignition source and eventually ignition occurs. In the second stage the fuel flow is continuously injected into the combustor, enabling the local ignition to propagate and spread in order to form stable combustion in the combustor. During the second stage, the engine speed is accelerated by increasing the fuel flow injection until the engine operates under a self-sustained speed.
As part of engine design testing procedures, gas turbine engines must be able to start under conditions involving a range of temperatures and altitudes. Altitudes can vary from as low as a few thousand feet below sea level to altitudes that are greater than 65,000 ft. above sea level. Temperatures can range from −60 degrees F. to +135 degrees F.
The typical engine fuel control will provide a fixed starting fuel flow for light-off based on the ambient pressure and ambient temperature. In the case of in-flight re-lights, the effects of ram may also be input into the fuel control system in order to further bias the fuel flow for light-off.
The engine's requirements for light-off fuel delivery vary significantly with engine size, the number of nozzles, the type of nozzles used, the altitude, the temperature of the air and fuel, the viscous and aerodynamic drag effects on the rotors and the forward velocity of the engine. Increasing altitude causes a rarefying of the air and a need to reduce the light-off fuel flow. Very cold temperatures cause a need for higher fuel flows in order to achieve light-off. In particular, at very cold, high altitude static starts, a high light-off fuel flow requirement may be needed in order to initiate light-off. However, sustaining this flow may result in overtemperatures in the turbine area and the associated stresses that follow. In addition, this high fuel flow required for light-off may lead to visual flame being emitted from the jet pipe or exhaust of the engine. Longer light-off times may result in fuel pooling. Once this pooled fuel finally burns, visible torching may result. This torching is highly undesirable as it may also lead to engine distress on the turbine blading.
Efforts have been made in the industry to improve gas turbine engine starting, particularly for reducing light-off time which is taken from the point of fuel injection to the light-off occurrence, in order to have a quick start-up of the engine. U.S. Pat. No. 5,718,111, issued to Ling et al. on Feb. 17, 1998, as an example of such efforts, describes a gas turbine engine start-up control system and method in which the engine exit temperature and the compressor speed change rate are sensed, and the sensed parameters are compared with desired start-up characteristics and referenced to look-up tables for determining an output composition factor. Based on the output composition factor, the start-up of the gas turbine engine is adjusted, generally by adjusting fuel flow through use of a fuel control system. However, this method and system are generally used for controlling fuel flow in the second stage of engine start-up because the engine exit temperature changes after the light-off occurrence.
U.S. Pat. No. 6,062,016, issued to Edelman on May 16, 2000 discloses a gas turbine engine light-off system and method in which the gas turbine engine is operated at a fixed speed in order to provide a substantially constant supply of combustion air for light-off, and the fuel flow is ramped up to achieve the correct fuel-to-air ratio for light-off.
U.S. Pat. No. 5,107,674, issued to Wibbelsman et al. on Apr. 28, 1992 describes a starting system for a gas turbine aircraft engine. The starting system automatically controls the sequencing of events needed during engine start-up that lead up to light-off, including sensors for ambient temperature, exhaust gas temperature, compressor speed, fuel flow, etc. The control schedules fuel flow in a manner which avoids stalls and takes corrective action when stalls occur, and provides scheduling of fuel flow in severely cold conditions.
U.S. Pat. No. 5,369,948, issued to Vertens et al. on Dec. 6, 1994 describes a process and apparatus for starting a gas turbine engine. In this apparatus with a start-up controller for a gas turbine engine, the amount of fuel injected by controlled dosing pumps is determined, whereby the amount of fuel injected can be regulated as a function of the difference between the injection pressure and the compressor pressure in the combustion chamber of the turbine, or at the compressor outlet.
Nevertheless, there is still a need for a better engine starting method for reducing light-off time of engine start-up and for avoiding injection of excess fuel during engine start-up, particularly under cold weather conditions because the fuel accuracy of achieving light-off of gas turbine engines becomes more critical as the fuel and air temperature get colder.